It is well known that coal carries surface moisture which occurs naturally or is present as a result of coal processing techniques, such as crushing and washing operations. The surface water may have been sprayed onto the coal to reduce dust, or may be present due to the fact that the coal is in the form of a slurry in water to facilitate transportation in conduits or on belts. Alternatively, the coal may have been exposed to a flotation process in which the coal is carried on the surface of a froth of water and chemicals.
There are two important reasons why residual water should be removed from cleaned coal prior to delivery to a user. The first reason is that surface moisture in excess of about 4 or 5 weight percent can result in serious freezing of coal in railroad cars when shipment is made in sub-freezing weather. The reason for this is that when surface moisture on particulate solids such as coal particles freezes, the ice acts as a powerful adhesive holding the particles together in a mass. The adhesivity is influenced by both the particle size of the solids and the moisture content. For example, coal with as little as 4% moisture will, when frozen, cohere so strongly as to require special handling to break up the frozen mass. It thus becomes difficult to unload or dump railway cars, trucks and other conveyances used to transport coal, mineral ores and other finely divided solids. It also makes it difficult to move coal out of outdoor cold storage piles in a condition for use as a fuel or other use. Unloading frozen coal from railroad cars is time-consuming, and can result in blocked dump shutes. This can result in as much as 30 to 60 tons of coal being left in a railroad car. Various techniques such as vibration, steam lances, fires under the cars, infrared heating in warming sheds and even dynamiting have been tried to unload cars containing frozen coal. The safety problems inherent in some of those techniques are obvious. Other techniques are ineffective or totally impracticable from an economic standpoint, especially where conditions are so severe as to cause entire car loads of coal to freeze solid, as opposed to freezing merely in the perimeter area. All of these factors point to the need for the development of an economic method for treating coal, ores, and other divided solids to overcome the problems of transport of those solids in cold conditions.
The second important reason for removing residual amounts of water is that any moisture which is present on the coal acts to reduce the B.T.U. fuel value of the coal. The fuel value of clean coal is an inverse function of its moisture content, and so any moisture present on the coal acts as a B.T.U. "thief".
In the past, dewatering of wet coal has been accomplished by procedures such as screening, filtration, centrifugation, forced air drying and thermal drying. These processes reduce the quantity of water on the coal, but vary in their effectiveness.
Numerous other approaches have been used with limited degrees of success. Sodium chloride and calcium chloride salts have been added to moist coal as it is being loaded, and this has resulted in some degree of success towards reducing the freezing problem. However, such salts contribute to corrosion of the equipment with which they come into contact. Oil has been used to freeze-proof coal but with questionable effectiveness. Oil soluble surfactants have been added to coal and seem to improve the results. Ethylene glycol has also been employed, but the success has been mitigated by the cost of the treatment which is very high.
Dewatering aids have been utilized in instances where the cost effectiveness of the chemical additives exceeds the incremental equipment charges and related energy costs. A commonly employed dewatering aid comprises a surface active chemical species known generically as sodium dialkylsulfosuccinate. However, that material exhibits a significant foaming tendency and has consequently enjoyed limited use in dewatering compositions.
Some of these problems are described in U.S. Pat. No. 4,426,409 to Roe. Other patents concerned with these problems are U.S. Pat. Nos. 4,039,466 to Matsuda et al, 4,231,868 to Wang et al, 4,153,549 to Wang et al, 4,206,063 to Wang et al, 4,207,186 to Wang et al, 4,191,655 to Quinn et al, 4,410,431 to Roe, 3,977,982 to Hertl and 3,943,061 to Cziska et al.